An internal mechanism regulating mechanical activity of muscles has been demonstrated by recording the effect of passive changes in length during contractures and twitches. The results are valid for all muscles which have been used, including isolated frog and rabbit cardiac muscle and frog skeletal muscles. It will be attempted to obtain more information on the nature and significance of this mechanism by studying the effect of length changes under different mechanical conditions and by determining whether and how the responses are affected by drugs and other chemical agents which are known to change contractility. Furthermore the implications of these results for the energetics of muscle will be tested by measuring heat production. In another proposed study, the generally accepted hypothesis that the speed of relaxation is controlled by the internal Ca concentration will be tested by measuring heat production during relaxation in muscles in which the speed of relaxation can be changed by chemical agents and other conditions. According to the Ca hypothesis slowing of relaxation should be associated with increased heat production. The work will be extended to smooth muscle, with particular attention to its mechanical properties. In the relaxed state this type of muscle is characterized by a slow stress relaxation. The significance of this property for the slowness of relaxation and for tonic states will be investigated.